Advances in current monolithic integration technology have allowed lasers of a variety of geometries to be fabricated, including V-shaped lasers and triangular ring lasers, as described, for example, in Applied Physics Letters, 59, pp. 1395-97, 16 Sep. 1991. These developments expand the prospective applications for integrated semiconductor lasers and add the attractiveness of greater manufacturability and reduced cost. This technology has opened the opportunity to explore new and novel features that can be combined inside and outside the laser cavity.
Copending U.S. patent application Ser. No. 10/226,076, filed Aug. 23, 2002, entitled “Wavelength Selectable Device” and assigned to the assignee hereof, the disclosure of which is hereby incorporated herein by reference, discloses monolithic structures that prevent back-reflection from the entrance facet of an element such as an electroabsorption modulator (EAM) into a laser cavity serving as a light source for the element by appropriate selection of the geometry of the device. Unidirectional emitting lasers would be desirable in such configurations to maximize the coupling of laser light into an EAM or other such elements.
As is known, a ring cavity laser possess benefits that a Fabry-Perot cavity does not have; for example, a ring cavity will produce lasing action with higher spectral purity than can be obtained with a Fabry-Perot cavity. Monolithic triangular ring lasers as well as other types of ring lasers and their integrated couplings have been described in the prior art, as well as in copending U.S. patent application Ser. No. 09/918,544 filed Aug. 1, 2001, entitled “Curved Waveguide Ring Lasers,” now U.S. Pat. No. 6,680,961, issued Jan. 20, 2004 and assigned to the assignee hereof, the disclosure of which is hereby incorporated by reference. In addition, unidirectional behavior in ring lasers also has been described in the prior art.
Over the past few years, thanks mainly to the popularity of the Internet, the demand for bandwidth has experienced explosive growth. Carrier companies and their suppliers have addressed this demand by installing Wavelength Division Multiplexing (WDM) systems which allow multiple wavelengths of laser light to be transmitted through a single strand of optical fiber. One of the main requirements of lasers used in WDM systems is that the laser sources have spectral purity, with the result that most of the power of the laser must be concentrated in a narrow wavelength range. A laser source has a large number of possible longitudinal modes, and although it has the tendency to operate in the one longitudinal mode that leads to the maximum gain, some other modes are also partially amplified, causing it to generate optical radiation in different wavelengths and reducing its spectral purity.
The use of a gap for improvement of the spectral characteristics of a Fabry-Perot laser that operates with standing waves was proposed by Larry A. Coldren and T. L. Koch, “Analysis and Design of Coupled-Cavity Lasers—Part 1: Threshold Gain Analysis and Design Guidelines,” IEEE Journal of Quantum Electronics, Vol. QE-20, No. 6, pp. 659-670, June 1984. As described, a gap was introduced between two cleaved facets to achieve coupling between different cavities, resulting in an improved mode discrimination depending on the lengths of the cavities and the gap. However, due to the inherent difficulty in building accurately positioned cleaved facets and the tolerances involved in placing two cleaved cavities in close proximity to each other to form a gap, large-scale manufacture of these lasers did not materialize. Etched mirror and groove coupled devices were demonstrated by Larry A. Coldren, et al., in “Etched Mirror and Groove Coupled GaInAsP/InP Laser Devices for Integrated Optics,” IEEE Journal of Quantum Electronics, Vol. QE-18, No. 10, pp. 1679-1688, October 1982. However, these etched facets where not equivalent in reflectivity to cleaved facets and the etched versions of these devices were less efficient than their cleaved counterparts. U.S. Pat. No. 4,851,368 taught a process that allowed etched laser facets to be fabricated that were equivalent in reflectivity to cleaved facets.